Sun’s nearest stellar neighbor may have Earth-like planet

Its solar system is only 4.2 light-years from ours

Earth may have a kindred planet no farther away than the star next door.

It’s a world at least 1.3 times as massive as Earth that orbits a dim star 4.2 light-years away. That star, alled Proxima Centauri, is the one closest to our sun. Its newly discovered planet whips around its star so fast that each year on that world lasts a mere 11.2 days. That reflects the fact that this planet is so close to its sun — just 5 percent as far as Earth is from our own sun. Yet that other world is at just the right distance from its star for any liquid water to still be able to flow on its surface.

Here are the relative sizes of Jupiter and some stars. Notice that Proxima, a runt of a red dwarf, is only slightly larger than Jupiter and far smaller than our yellow sun, third from the top.

ESO

That makes this exoplanet — a world outside our solar system — the closest known.

"It’s not clear if the planet will be Earth-like,” says Guillem Anglada-Escudé. This astronomer at Queen Mary University of London, England, led the exoplanet’s discovery team. As the nearest planet outside our solar system, this newfound Proxima b (not the most creative of names for a planet) is an attractive target to learn about alien atmospheres. It also may be a great place to hunt for signs of extraterrestrial life.

Anglada-Escudé’s team offered the first report on this planet August 25 in Nature.

As star’s go, Proxima Centauri is a runt. It lies in the southern constellation Centaurus. Temperatures on its surface run about 2,800 degrees Celsius (5,000 degrees Fahrenheit) cooler than on our sun. The result: Proxima glows a feeble red. The star’s size is much closer to that of Jupiter than our sun. And despite it being so close, Proxima cannot be seen with the naked eye. That’s why it wasn’t discovered until 1915.

Proxima Centauri is part of a triple-star system known as Alpha Centauri. Four years ago, astronomers reported in Nature that another star in this group — Alpha Centauri B — hosts a planet roughly as massive as Earth. That world, however, would likely be too hot to host life. But not all scientists accept that such an exoplanet even exists there. Researchers last year reported in the Monthly Notices of the Royal Astronomical Society Letters that they could find no evidence for the planet.

But Proxima’s planet seems real. And if it is, “It’s an incredible discovery; it’s almost a gift,” says David Kipping. He’s an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

How they found the new planet

Anglada-Escudé’s group found its quarry by looking for a super tiny wobble in the position of its star. That wiggle would be due to a gravitational tug by the orbiting planet. For two months in early 2016, the astronomers led an intensive viewing campaign to follow up on early hints of a planet. They used two instruments in Chile. One was the European Southern Observatory’s 3.6-meter telescope. The other is known simply as the Very Large Telescope.

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This shows the tiny wobble in Proxima Centauri’s motion — back and forth, relative to Earth — in the first half of 2016. The pattern repeats every 11.2 days, consistent with the gravitational tug of the planet as it orbits around its star.

ESO/G. ANGLADA-ESCUDÉ

Details about our not-too-distant neighbor remain skimpy. There’s no data yet on what its atmosphere is like. Even its precise size remains uncertain. And although it is just one star away, “we will likely have to wait a long time in order to learn anything more about the planet,” says Heather Knutson. She’s a planetary scientist at Caltech in Pasadena, Calif.

For now, Knutson says, the best bet is to hope that the planet, when viewed from Earth, passes in front of Proxima Centauri. That would allow its starlight to filter through the planet’s atmosphere. Gases in that atmosphere would betray their presence by absorbing specific colors of light. And they could point to more than just what makes up that atmosphere. The presence of oxygen, methane and carbon dioxide, for instance, are widely considered chemical markers of life.

NASA’s James Webb Space Telescope is due to launch in late 2018. If the exoplanet does pass in front of its star, that space telescope should be able to detect its atmosphere, says Mark Clampin. He's an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. Hundreds of hours of telescope time would need to be dedicated to the task. "It will be an extremely challenging observation, but not impossible," he says.

Scientists can estimate the newfound planet’s size by measuring how much light it blocks as it passes in front of its star. That size, combined with its mass, could tell researchers something about how dense it is. At issue is whether this planet is puffy and gassy like Jupiter or rocky like Earth.

Kipping has already been monitoring Proxima Centauri with Canada’s MOST satellite. He’s been looking for a telltale periodic dip in the star's light. That dip would be caused by the planet partially blocking its star. There’s only a 1.5 percent chance, however, that the planet lines up just-so with its star. And there’s a second problem. The light from Proxima Centauri tends to vary somewhat. That will make any additional drop in brightness from the planet passing in front of it hard to detect.

If things are not lined up just right, gaining more data on the planet will be “much more difficult,” Knutson says. Astronomers would have to rely on light coming from the planet. That would have to be either an intrinsic infrared glow (of heat) or some visible light reflected from its sun. The James Webb Space Telescope might be able to barely sense infrared light coming from Proxima b. But it could be at least a decade before any other instrument is up to the challenge.

And even then, there are no guarantees. “It’s going to be very difficult to characterize the planet without sending a probe there,” Kipping says. By probe he is referring to a spacecraft.

Planning a visit to the new world

A research team hopes to send out just such a probe. Many probes, actually. Their project is known as Breakthrough Starshot. In April, it announced a plan to put $100 million toward developing new technology that could send a fleet of tiny nanocrafts toward Alpha Centauri. Each robotic probe would weigh just a few grams. Researchers would nudge them on their way using an Earth-based 100 gigawatt laser.

The goal would be to accelerate them to roughly 20 percent the speed of light. If successful, this armada might be able to reach Alpha Centauri within 20 years of its launch. Quite a boost in technology would be needed to do this. For instance, it would take the fastest spacecraft ever to leave Earth — the New Horizons mission to Pluto — nearly 90,000 years to complete this journey. And that’s if it continued traveling at its current speed of about 52,000 kilometers (32,000 miles) per hour.

The new announcement by Anglada-Escudé’s team “is likely to energize the [Breakthrough Starshot] project,” says Avi Loeb. He’s an astrophysicist at Harvard and chairman of Breakthrough’s advisory committee. The goal would be to send back photos taken by a camera and various filters. Ground-based researchers could then use them, Loeb says, to infer whether that planet “is green (harboring life as we know it), blue (with water oceans on its surface) or just brown (dry rock).”

If anything is alive on the planet, it likely would prove quite different from anything on Earth. Organisms that photosynthesize — turn light into food — would have to deal with a faint, cool star. That star emits mostly heat, not bright light. Proxima Centauri also is known to host exuberant flares. They would buffet any near-orbiting planet with bursts of potentially killer conditions: ultraviolet radiation and X-rays. As a result, “conditions on such a planet would be very interesting for life,” says Lisa Kaltenegger. She works as an astrophysicist at Cornell University in Ithaca, N.Y.

Scouting for alien life

Given such an alien environment, life might show up in unusual ways. Kaltenegger and Cornell astronomer Jack O'Malley-James propose looking for a glow. They call this glow biofluorescence (BY-oh-flor-ESS-ents). It would come from some of the planet’s organisms. They envision it might be triggered by the ultraviolet light emitted following those periodic flares by its sun.

Critters on Proxima b might have evolved biofluorescence as protection. They might be able to transform harmful ultraviolet light into more comfortable visible light. And this could show up as a flicker of light picked up by an Earth-based telescope.

The Pale Red Dot campaign was an international search for an Earth-like exoplanet around Proxima Centauri. Social media posts kept the public informed about how teams of astronomers in different fields worked together to collect, analyze and interpret data. This ultimately confirmed the presence of a nearby exoplanet.

ESO/Pale Red Dot

“The idea that we could spot a glow seems to be right out of a [science fiction] novel,” admits Kaltenegger. She described the concept online August 24 on arXiv.org.

That all assumes that something could survive on the planet. If Earth were placed in the same orbit as Proxima b, it would lose its protective stratospheric ozone layer roughly three times every Earth-year, Kipping says. Obviously, he adds, “That’s kind of bad.” That would be too fast for the atmosphere to build its ozone layer up again.

But if life has taken shelter underground or underwater — or doesn’t need oxygen — it still might survive.

Whether or not critters crawl on Proxima b, the discovery of the planet “could really usher new energy into the search for other nearby worlds,” says Margaret Turnbull. This astronomer works with the SETI Institute in Madison, Wisc. Most exoplanets are hundreds to thousands of light-years away. But little is known about the possible planet families huddled up to the stars nearest to us. “I’d love to see interstellar travel,” says Turnbull. “To really inspire that kind of effort, we need interesting destinations like this.”

Power Words

arXiv A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.

astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atmosphere The envelope of gases surrounding Earth or another planet.

carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide (or CO2 ) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H 2 O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

constellation Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

exoplanet A planet that orbits a star outside the solar system. Also called an extrasolar planet.

fiction (adj. fictional) An idea or a story that is made-up, not a depiction of real events.

filter (in chemistry and environmental science) A device which allows some materials to pass through but not others, based on their size or some other feature. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

Goldilocks zone A term that astronomers use for a region out from a star where conditions there might allow a planet to support life as we know it. This distance would be not too close to its sun (otherwise the extreme heat would evaporate liquids). It also can’t be too far (or the extreme cold would freeze any water). But if it’s just right — in that so-called Goldilocks zone — water could pool as a liquid and support life.

host (in biology and medicine) The organism in which another lives. Humans may be a temporary host for food-poisoning germs or other infective agents.

interstellar Between stars.

Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

liquid A material that flows freely but keeps a constant volume, like water or oil.

magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.

marker (in biomedicine) The presence of some substance that usually can only be present because it signals some disease, pollutant or event (such as the attachment of some stain or molecular flag). As such, this substance will serve as a sign — or marker — of that related thing.

mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

methane A hydrocarbon with the chemical formula CH 4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

NASA See National Aeronautics and Space Administration

observatory (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.

orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

ozone A colorless gas that forms high in the atmosphere and at ground level. When it forms at Earth’s surface, ozone is a pollutant that irritates eyes and lungs. It is also a major ingredient of smog.

ozone layer A layer in Earth’s stratosphere. It contains a lot of ozone, which helps block much of the sun’s biologically damaging ultraviolet radiation.

planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Pluto A dwarf planet that is located in the Kuiper Belt, just beyond Neptune. Pluto is the tenth largest object orbiting the sun.

radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

science fiction A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds.

SETI An abbreviation for search for extraterrestrial life, meaning life on other worlds.

solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

speed of light A constant often used in physics, corresponding to 1.080 billion kilometers (671 million miles) per hour.

star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

stellar An adjective that means of or relating to stars.

sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.

technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

ultraviolet light A type of electromagnetic radiation with a wavelength from 10 nanometers to 380 nanometers. The wavelengths are shorter than that of visible light but longer than X-rays.

X-ray A type of radiation analogous to gamma rays, but of somewhat lower energy.